Compound exercises are multi-joint closed kinetic chain functional movements with several muscles working together. The compound exercises produce the greatest increases in muscle size and strength. This is due to the greater amounts of weight lifted and the large number of muscles involved. The movement of compound exercises consists primarily of pushing or pulling movements creating a substantially linear effect of distal motion such as in the hands or feet. However, the straight line movement is created by joints which can only rotate. The body can create a straight line movement by the combined movement of two joints rotating in opposite directions. This cancels out the rotational effect to form a straight line. Compound exercises include but are not limited to squat, bench press, press, row, dip, pull down, etc.
Isolation exercises or open kinetic chain exercises are typically single joint movements that isolate single muscle groups from the rest of the body using lighter weights than is possible with compound exercises. Isolation exercises are considered to be high in shear stress on the joints. Also, they are not considered functional exercises in that many of the movements do not transfer to daily activities in which the muscles work together; not in isolation from the rest of the body. One advantage of isolation exercises is that these easily allow the use of rotational resistance about a single pivot providing a full range of motion and even distribution of resistance not possible with conventional compound exercises.
Linear resistance creates shear stress caused by rotating joints opposing linear resistance. This creates both high shear force and sticking points when the limbs are perpendicular to the load as well as producing insufficient muscle stimulus during the exercise movement as resistance progressively diminishes due to improving mechanical advantage as they approach parallel to the resistance. Arthur Jones Nautilus machines provided a solution with machines that provided rotational resistance to the muscles by rotating the resistance coaxially to the same axes of rotation as the joints. Many single joint isolation machines now incorporate this method to provide rotational resistance to single muscle groups.
Attempts to solve the problems of compound exercises have focused on supplying resistance to the primarily linear distal movement of the hands or feet rather than to the movement of the joints creating that movement. This results in shear stress on the joints when the limbs are in a horizontal position relative to the load. Sled type leg press, single pivot lever arm machines, even 4 bar linkage machines as well as cable machines only produce a linear, convex, concave, or modified mixture of the two. Most of the foregoing conventional systems attempt to mimic the natural distal motion of the hands or foot movement through the exercise, rather than the movement of the joints during the exercise. Previous attempts to reduce shear stress focus solutions to the location of the discomfort rather than to the cause. Some of these attempted solutions include knee wraps and weight belts for squats, leg press machines in which the lumbar is pressed against a back rest with body maintaining a seated position and the legs extending out in front of the body, allowing the knee angle to be reduced to reduce knee shear stress. This compresses the body while not allowing the hips to fully extend and so can be uncomfortable as well as not a functional movement. A commonly advised solution is to use light weights. This works to reduce shear stress on the knees and lumbar but also removes most of the beneficial muscle stimulus.